1. Field of the Invention
The present invention relates generally to integrated circuit fabrication tools, and more particularly, relates to a method and apparatus to measure pellicle height.
2. Background Information
Photolithography is a process that is commonly used in the manufacture of integrated circuits. The process involves the deposition of a photoresist layer onto an underlying substrate layer. The photoresist is then selectively exposed to light, which chemically alters the photoresist. If the photoresist is a positive type, then areas where the photoresist are exposed to light are selectively removed order to obtain the circuit pattern.
The pattern that is projected onto the photoresist is contained on a mask placed within a photolithography exposure tool or stepper. A stepper includes a light source, stepper controller (e.g., an aligner), and a reticle table. A mask, also referred to as a reticle, is placed on the reticle table between the light source and the photoresist layer. The reticle is typically formed from an opaque film of patterned chromium placed on a transparent blank typically made of a glass or quartz material. The pattern is transferred onto the photoresist by projecting an image of the reticle onto the photoresist using an exposing radiation from the light source. In this manner, the pattern of opaque chromium placed on the transparent blank defines the areas through which the exposing radiation passes, thereby exposing selective regions of the underlying photoresist.
The patterned chromium film on the reticle blank includes lines and line spacings of less than 10 microns. Depending on a reduction factor, line width, and line spacing, geometries for a resulting semiconductor device can range from less than 10 microns to less than two microns. When working with such small geometries, it is important that the reticle and other components in the fabrication processes be free of contamination. Even the smallest piece of foreign contamination can produce an image on the photoresist.
Therefore, in many applications, the reticle is covered by a pellicle. A pellicle (sometimes referred to as a pellicle membrane) is a thin film of optical grade polymer that is stretched on a pellicle frame and secured to the reticle. The pellicle frame is often made of a rigid and durable material, such as aluminum, so that the fragile pellicle may be handled during transport or during the photolithography process without damage. The pellicle""s purpose is to prevent airborne contaminants from collecting on the mask. During the exposure, the pellicle holds the contaminants out of the focal plane, and hence, the contaminants do not print on the wafer.
Complex circuit patterns typically require multiple exposure and development steps. In some instances, multiple copies of circuit patterns are repeatedly projected onto different surface regions on a single wafer, thereby allowing several identical integrated chips to be manufactured from the same wafer. The wafer is then subsequently cut into a plurality of identical, individual chips. In the case of a large circuit pattern, sections of the pattern may be exposed and developed individually rather than the circuit pattern as a whole. The stepper controller controls the movement and positioning of the reticle as it moves across different regions of a wafer during the photolithography process.
Because of these complex or multiple patterns, reticles in the stepper often have to be repeatedly switched or removed during the photolithography process. Removal of a reticle may also be required if a xe2x80x9cworn outxe2x80x9d reticle (e.g., a reticle distorted in some manner due to damage, heavy use, or dirt accumulation) needs to replaced by another reticle.
Because the pellicle-reticle assembly is continuously being exposed to the light source and because of other factors present in the photolithography process, the pellicle (including the pellicle frame) and the reticle can heat up. The heat causes these components to expand, which can have very detrimental effects when these components are repeatedly removed from or inserted into the stepper as mentioned above. That is, the expanded size could undesirably result in contact between the pellicle and a component of the stepper (such as optical machinery) when the pellicle-reticle assembly is removed from the stepper. This contact can scratch fragile components of the stepper or damage the pellicle. Scratched stepper components can result in out-of-focus imaging or improper positioning of the reticle, which ultimately decreases die yield and productivity. A scratched pellicle can introduce contaminants onto the reticle image. Furthermore, replacement of a damaged pellicle and reticle assembly can be expensive, especially if several wafers are exposed and developed before the damage to the pellicle is uncovered.
Accordingly, there is a need to be able determine the height of a pellicle to determine if it has exceeded acceptable tolerances.
According to one aspect of the invention, a method for measuring a thickness of a pellicle is provided. The method includes projecting a light beam from a first side of the pellicle and at a plane above a first surface of the pellicle, projecting the light beam from the first side of the pellicle and at a plane corresponding to a plane of the pellicle, and projecting the light beam from the first side of the pellicle and at a plane below a second surface of the pellicle. The resulting intensity of the projected light beams is detected at a second side of the pellicle opposite to the first side, and an index corresponding to each detected resulting intensity is generated. The thickness of the pellicle is determined based on the generated indexes.